Radio Frequency Identification, which is a non-contact automatic identification technology, identifies target objects and obtains related data automatically through RF signals, featuring that the identification can be conducted in various harsh environments without the necessity of manual intervention. The RFID is a wireless system which comprises an interrogator (or a Reader) and many transponders (or Tags). When the antenna used on the tag is attached to the surface of metal materials, the identification rate of common tags is unstable at different positions. Furthermore, different metal environments have to be taken into consideration.
What is different from the Reader of the passive radio frequency identification is that the RFIT is used by attaching it to objects of various materials and in different shapes, so the use of the existing RFIT has limit of use in a metal environment, while the characteristic of stable identification is often the basic design requirement of the tag antenna.
In the radio frequency identification technology, to maximize the function of the reading range, the proper design of the RFIT antenna is especially important. Therefore, the transmission of maximum electric power to the tag chip without loss is required, to the fullest extent possible. For this purpose, the RFIT antenna shall be well matched with the tag chip while having good radiation characteristic. Usually, the RFIT chip has the function of recording various information and reading the information saved.
When the RFIT antenna is attached to a metal body, the return loss and radiation pattern characteristics of the antenna are sensitively affected. Therefore, attention shall be paid to this fact in the antenna design. Generally, a dipole antenna brought near a metal body cannot radiate electromagnetic waves, while, according to the reflected wave on the metal surface, the electric power required by the driving of RFIT chip cannot be ensured. In addition, as the components of the parasitic capacitance between the metal surface and the antenna change, the characteristics, such as the resonance frequency, antenna, impedance and radiation efficiency, will also change.
Therefore, as for the existing identification tag used before, the tag antenna is made by inserting various materials between the metal body and the tag antenna to keep a certain spacing between the radio frequency identification tag and the metal. However, the application of tag antennas made in this way in practical commonly-used products brings some inconvenience. In different environments, such as different metal bodies and different shapes, there are different requirements for such tag antennas, which is not advantageous for mass production. Furthermore, the use environments may cause damage to tags, especially when increasing of the thickness and size, which is not advantageous for miniaturization.
Therefore, an antenna using a metal body as its ground shall be considered to be a metal-body-attaching tag antenna.
The well-known typical antennae of such category are just microstrip patch antennae and planar inverted-F antennae (PIFA).
Usually, although the manufacturing of the microstrip patch antenna is relatively easy, the tag antenna made seems a little bigger due to the half-wave design on resonant frequency, which becomes the disadvantage. Compared to the microstrip patch antenna, the quarter-wave design of the PIFA antenna on resonant frequency makes miniaturization possible, but the consequent problem is that manufacturing becomes complex. Furthermore, when the antenna is attached to a metal body, the resonance characteristic will have some changes as materials and shapes of metal surfaces vary.
In view of this, provision of an RFIT and an antenna thereof to solve the problem above has become a technical problem in need of urgent solution in the field.